Christopher L. Stevenson

Modeling an inhomogeneous optically thick laser induced plasma: a simplified theoretical approach ☆

Abstract A simplified theoretical approach is developed for an optically thick inhomogeneous laser induced plasma. The model describes the time evolution of the plasma continuum and specific atomic emission after the laser pulse has terminated and interaction with a target material has ended. Local thermodynamic equilibrium is assumed allowing the application of the collision-dominated plasma model and standard statistical distributions. Calculations are performed for a two-component Si/N system. The model input parameters are the number of plasma species (or plasma pressure) and the ratio of atomic constituents. Functions are introduced which describe the evolution of temperature and size of the plasma. All model inputs are experimentally measurable. The model outputs are spatial and temporal distributions of atom, ion and electron number densities, evolution of an atomic line profile and optical thickness and the resulting absolute intensity of plasma emission in the vicinity of a strong non-resonance atomic transition. Practical applications of the model include prediction of temperature, electron density and the dominating broadening mechanism. The model can also be used to choose the optimal line for quantitative analysis.

Laser-Excited Synchronous Luminescence Spectroscopy

The use of lasers as excitation sources for molecular luminescence often results in improvements in sensitivity and limits of detection (LODs). Synchronous luminescence (SL) spectroscopy, in which both excitation and emission wavelengths are scanned simultaneously, provides a convenient means to improve selectivity (often dramatically) in the analysis of multicomponent mixtures using room-temperature luminescence. We report here on the first use of a dye laser as an excitation source for SL at room temperature. The performance of the laser synchronous luminescence (LSL) system is described for the analysis of four polyaromatic compounds; for one of these—tetracene—the LOD was 680 zeptomoles (10−21 mol) in the volume probed by the laser. In addition to impressive sensitivity and selectivity, the laser system used is quite small and can be considered as an attractive source for portable SL instruments designed for in-field screening of environmental samples.

Analysis of polynuclear aromatic compounds using laser-excited synchronous fluorescence

Abstract Monitoring the presence and distribution of polynuclear aromatic compounds (PACs) in the environment is an important task due to the presence of these species in many types of environmental samples. Molecular fluorescence is often used for this task, usually after extensive sample cleanup and separation, due to the speed and the inherent sensitivity of the technique for PACs. When synchronous fluorescence is used for PAC analysis, the amount of sample pretreatment can often be reduced due to the greater selectivity of the technique relative to conventional fluorescence emission. Another method of increasing sensitivity and selectivity is to use lasers as the excitation source in molecular fluorescence. An added advantage of lasers is the high source coupling and fluorescence collection efficiencies when fiber optics are used for remote sensing applications. In this work, the applicability of laser-excited synchronous fluorescence for the analysis of PAC mixtures is evaluated. A prototype laser-excited synchronous luminescence (LSL) instrument has been previously determined to offer significant advantages over conventional laser-excited fluorescence [C.L. Stevenson and T. Vo-Dinh, Appl. Spectrosc., 47 (1993) 430]. An improved version of the LSL instrument is presented here with superior scanning precision and extended wavelength scanning range. In addition to demonstrating the capabilities of the improved LSL instrument for mixture analysis, the sensitivity of this improved instrument is compared to that of a commercial fluorimeter. Finally, the capability of the LSL instrument to further improve selectivity using time-resolved synchronous fluorescence is illustrated.

Fast Scanning Synchronous Luminescence Spectrometer Based on Acousto-Optic Tunable Filters:

A new luminescence spectrometer based on quartz-collinear acousto-optic tunable filters (AOTFs) and capable of synchronous scanning is described. An acousto-optic tunable filter is an electronically tunable optical bandpass filter. Unlike a tunable grating monochromator, an AOTF has no moving mechanical parts, and an AOTF can be tuned to any wavelength within its operating range in microseconds. These characteristics, combined with the small size of these devices, make AOTFs an important new alternative to conventional monochromators, especially for portable instrumentation. The relevant performance of the AOTFs (efficiency, bandwidth, rejection, etc.) is compared with that of typical small-grating monochromator.

Two-step laser excited atomic fluorescence spectrometry determination of mercury

Abstract A novel method for the determination of mercury by laser excited atomic fluorescence with electrothermal atomization (LEAFS-ETA) has been developed. The experimental set-up consisted of a dual dye-laser system pumped with a XeCl excimer laser operated at 10 Hz, and an electrothermal atomizer with platform atomization. The atomization program allowed time for the injection of Pd (as a matrix modifier) and used a drying step at 110°C and an atomization step at 1200°C. The collection is made at 90° using a pierced mirror, an achromat lens and a long-pass filter. The monochromator is fitted with a 1P28 PMT. The signal is processed by using a boxcar and an analog to digital interface. The excitation scheme is a two-step process, with λ1 = 253.7 nm and λ2 = 435.8 nm. Direct fluorescence is observed at 546.1 nm. The limit of detection (LOD) obtained is 90 fg (9 pptr with 10 μ1 injection). The linear dynamic range (LDR) is five orders of magnitude and is limited by the non-linearity of the co-operative processes occurring at higher concentrations. In order to extend the LDR to higher amounts of mercury, indirect fluorescence is collected with the less sensitive line at 407.8 nm, allowing concentrations of 1 ppm and up to be measured, extending the LDR of the technique to at least seven orders of magnitude.

Estimating Detection Limits in Ultratrace Analysis. Part I: The Variability of Estimated Detection Limits:

The limit of detection (LOD) of a trace analytical method is often used as a gauge of the sensitivity of the method. Improvements and comparisons of similar methods usually involve comparisons of the LODs. However, the calculated LOD value is sensitive to a number of factors such as the method of calculation used, the choice of calibration standards, and the number background (blank) measurements. In this paper, the calculated LOD of a particular analytical method is viewed as an estimate of the true, unknown LOD. Factors which affect the variability of the estimate are investigated. Knowledge of the variability of the estimated LOD can help when analytical methods are being compared on the basis of LOD values. In addition, by knowing which experimental factors contribute to the uncertainty in the calculated LOD, it is possible to determine the analytical protocol which estimates the true LOD most efficiently with no loss in precision.

Ground state saturated population distribution of OH in an acetylene-air flame measured by two optical double resonance pump-probe approaches

Two optical double-resonance pump-probe techniques were used to determine the ground-state rotational population distributions of OH in an acetylene-air flame when a saturating laser beam is tuned to the Q(1)4 transition of the (0, 0) Sigma-II band. The saturated absorption technique is based on the detection of absorption by a probe laser under conditions of saturation with a pump laser and no saturation. In the fluorescence technique, a probe laser is scanned through the (1, 0) band, while a saturating pump laser, tuned to the (0, 0) band, is on or off. We found that approximately 15% of the total population of the ground state was transferred to the excited state. Perturbation of the rotational population distribution was greater for rotational levels close to the directly excited laser-coupled level. The rotational energy transfer rate in the ground state was somewhat greater than in the excited state. The assumption of the balanced cross-rate model was verified as a means of determining the absoslute OH number density with adequate accuracy.

Estimating Detection Limits in Ultratrace Analysis. Part II: Detecting and Counting Atoms and Molecules

Although the limit of detection (LOD) is a widely used means of indicating the detection power of an analytical technique, the application of conventional detection limit theory is not straightforward with respect to laser spectroscopic methods which are capable of detecting single atoms or molecules in the laser beam. In this paper, theoretical considerations in the detection and evaluation of these methods are addressed on the basis of a simple model of a typical laser spectroscopic experiment; in addition, the theoretical requirements for the precise counting of atoms, and factors which influence the signal variance in the model, are considered. Simple Monte Carlo computer simulations are used to verify and demonstrate the application of the theory of single atom detection (SAD) to typical experimental situations.

Laser-excited atomic-fluorescence spectrometry with electrothermal tube atomization

The performance of graphite-tube electrothermal atomizers is evaluated for laser-excited atomic-fluorescence spectrometry for several elements. Three pulsed laser systems are used to pump tunable dye lasers which subsequently are used to excite Pb, Ga, In, Fe, Ir, and Tl atoms in the hot graphite tube. The dye laser systems used are pumped by nitrogen, copper vapour and Nd:YAG lasers. Detection limits in the femtogram and subfemtogram range are typically obtained for all elements. A commercial graphite-tube furnace is important for the successful utilization of the laser-based method when the determination of trace elements is intended, especially when complicated matrices may be present.

Fluorescence dip spectroscopy of sodium atoms in an inductively coupled plasma

Abstract Two dye lasers were used to pump sodium atoms present in an inductively coupled plasma to the 4 d levels via 3 s -3 p excitation. By monitoring the fluorescence of both yellow lines while scanning the second laser, two dips were observed when the second laser was tuned to 568.263 nm ( 2 P 1 2 - 2 D 3 2 )and to 568.820 nm ( 2 P 1 2 - 2 D 5 2 , 3 2 ). The plots of the reciprocal relative fluorescence dips as a function of the reciprocal spectral energy density of the second laser (second excitation step) resulted in straight lines with slopes proportional to the quantum efficiencies of these transitions and therefore to the spontaneous transition probabilities. The experimental ratio between the gA values obtained by this method is compared to that calculated from tabulated values. In addition, the absolute values of excited state quantum efficiencies derived from these plots as well as from the saturation curves of the fluorescence dips are discussed in terms of possible ionization losses from the 4 d levels.